A significant physical property of gasoline is its volatility, or its ability to combust. There are two physical properties that are primarily used to measure the volatility of gasoline: (1) vapor pressure, and (2) vapor-liquid ratio. The Reid method is the standard test for measuring the vapor pressure of petroleum products. Reid vapor pressure is related to true vapor pressure, but is a more accurate assessment for petroleum products because it considers sample vaporization as well as the presence of water vapor and air in the measuring chamber.
In order to keep cars performing at a consistent level year-round, gasoline marketers blend volatility-modifying agents such as butane with gasoline to increase the volatility of the gasoline, especially during the colder months. The addition of butane also reduces the cost of the gasoline, since the unit cost of butane is typically less than the unit cost of gasoline. Butane can be added to the gasoline supply at several points along the gasoline distribution chain. The first opportunity to blend butane with gasoline is at the refinery, according to processes like those disclosed in Mayer, U.S. Pat. No. 3,751,644. This patent, which is owned by Sun Oil Company, describes a system for automatically adjusting the amount of butane added to a gasoline stream at a petroleum refinery based on continuous measurements of the Reid vapor pressure of the gasoline downstream from the point of blending. This downstream feedback control system adjusts the amount of butane added to the gasoline based on measurements taken downstream of the blending operation, and does not include measuring the volatility upstream of the blending operation, or calculating the blend ratio based on the upstream volatility.
Bajek's U.S. Pat. No. 3,999,959, which is owned by Universal Oil Products Company, also discloses a system for blending butane and gasoline at a petroleum refinery. The Bajek system blends butane with a low-octane gasoline stream and a high-octane gasoline stream, and then analyzes the blended gasoline to measure characteristics such as Reid vapor pressure and vapor to liquid ratio. Bajek does not disclose monitoring the gasoline upstream of the blending operation, or calculating the blend ratio based upon such upstream monitoring.
Butane is also added to gasoline while it is transported in the pipeline, after consolidation of various trunk lines from refineries. The inventors are aware of an unpatented system that is used to blend butane and gasoline at several terminal tank farms. These systems continuously monitor the Reid vapor pressure of gasoline that is introduced to a storage tank, and blend butane with the gasoline based upon the vapor pressure measurements. The butane is supplied by a tanker truck, as opposed to a permanent on-site mounted butane bullet. These unpatented systems do not automatically vary the blending process based upon the time of year, petroleum destination, or type of gasoline, but instead require manual oversight and control of the process. These unpatented systems also do not continuously monitor the Reid vapor pressure downstream of the blending operation as an integrity check. Instead, they certify the integrity of the blending operation by periodically measuring the Reid vapor pressure of the entire storage tank.
The third point of blending is at the tank farm, e.g., when gasoline is delivered to a large storage tank, the volatility of the tank is measured, and sufficient butane is added to the tank to attain a desired volatility. Butane can also be added immediately before the gasoline is dispensed to a tanker truck at the rack, or before gasoline is loaded onto a ship. Methods of blending butane into various gasoline streams are described by the present inventors in U.S. Pat. Nos. 6,679,302 and 7,631,671.
Because gasoline vapor emissions are a major component of volatile organic compounds (VOC's) in the atmosphere, the United States Environmental Protection Agency (EPA) has promulgated regulations that govern the maximum volatility of gasoline based on geographic regions in the United States during particular times of year. These regulations generally apply from May 1 through September 15, when the gasoline is warmest and most volatile, and require that any blended gasoline be certified for compliance with the volatility guidelines. See 40 C.F.R. §80.27. EPA has also promulgated regulations that prescribe the methods for testing blended gasoline for volatility. In particular, these regulations require that volatility be determined using American Society for Testing and Materials (ASTM) standard test method ASTM D5191. See 40 C.F.R. §§80.82 and 80.46.
Existing methods of enriching gasoline with butane generally monitor volatility upstream of the blending operation, determine the ratio that gasoline and butane can be blended together to achieve a desired vapor pressure, and blend the gasoline and butane at the blend ratio. Although these methods and systems are precise, they are inadequate for making batches of butane-enriched gasoline, especially during the summer months, because of the need to certify the volatility of the batch after production using manual testing techniques. Further, the existing methods and systems suffer from a substantial lack of automation, and methods of tailoring the blending operation to produce batches that meet the particular volatility requirements of a geographic region during any given time of the year.